Conventionally, in order to restrain the generation of radio-wave noise caused by spark discharges of a spark plug, a resistor-incorporated spark plug in which a resistor is disposed between a center electrode and a metal terminal has been widely used. Under certain conditions of use, such a resistor-incorporated spark plug has exhibited the emergence of a phenomenon of increase in resistance of the resistor due to oxidation of carbon contained in the resistor caused by electric energy which flows through the resistor. In order to cope with the phenomenon, for example, Patent Documents 1 and 2 mentioned below disclose restraint of oxidation of carbon through addition of metal powder having reducing action to the resistor for improving under-load life.